Laser applications often require a work piece to be irradiated with two or more individually controlled laser beams. Prior art methods of providing such a plurality of individually controlled laser beams have involved the use of arrays of beamsplitters including polarization-sensitive beamsplitters and polarization rotators. Using such beam splitter arrays together with separate modulators or controllers, while less costly than using a separate lasers for each required laser beam, may still prove prohibitively expensive, depending on a particular application.
Another method involves dividing a beam using an acousto-optic deflector powered at a number of different frequencies equal to the number of beams desired. This method is described in U.S. Pat. No. 7,003,003 assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated by reference. An abbreviated description of the method disclosed in the '003 patent is set forth below with reference to FIG. 1.
FIG. 1 schematically depicts beam-dividing apparatus 10 in which there are laser beam paths and connections between electronic and electrical components. Beam paths are depicted by fine lines, and electrical connections are depicted by bold lines. Apparatus 10 includes a carbon dioxide laser CO2 laser 12 including an RF power supply (not shown). A CO2 laser can provide an output beam having a wavelength between about 9 and 11 micrometers (μm). A controller 14 controls the output power of the laser and commands the RF power supply to operate the laser in a selected mode such as continuous wave (CW) or pulsed mode. Laser 10 delivers an output beam 16. Beam 16 is directed by turning mirrors 18 and 20 into an acousto-optic cell 22. One preferred acousto-optic cell is a model AGD-406 available from IntraAction Corporation of Bellwood, Ill. Such an AO-cell is generally referred to as a broadband AO-cell. Broadband AO-cells are designed to maintain the Bragg relationship (see below) over the entire bandwidth of the device. This allows the cell to be simultaneously driven at a plurality of different RF frequencies and provides minimal variation of the diffraction intensity, for example, less than about 10%, across a wide range of possible diffraction angles.
In apparatus 10, AO-cell 22 is driven by RF voltages at four different RF frequencies, f1, f2, f3, and f4, within the bandwidth of the AO-cell. Each driving frequency deflects a portion of output beam 16 at a particular angle depending on the frequency. The power in each diffracted portion (diffracted beam or secondary beam) is dependent, inter alia, on the power in beam 16 and the magnitude of the driving frequency, i.e., the magnitude of the RF voltage at that driving frequency. The diffraction angle (the Bragg angle) is given by the Bragg relationship:Sin θBn=λ0fn/2N0Va  (1)where θBn is the Bragg angle for frequency fn, fn is the driving frequency; λ0 is the laser beam wavelength; N0 is the refractive index of the acousto-optic cell material at wavelength λ0; and Va is the acoustic velocity in the cell material. In this example, the diffracting material of the cell is germanium (Ge), which is transparent for output wavelengths of the CO2 laser. Those skilled in the art will recognize that other laser wavelengths may require a cell having a different diffracting material.
Acoustic waves propagated in the acousto-optic material of the AO-cell by the driving frequencies generate optical phase gratings (not shown) within the acousto-optic material, through which laser output beam 16 passes. The angular (frequency) resolution of AO-cell depends, inter alia, on the size of beam 16 at the AO-cell and the driving frequencies. Beam-size is adjustable by a telescope or beam expander 24. Alternatively, the driving frequencies can be varied, to increase or decrease the spacing of the phase gratings.
In apparatus 10, driving frequencies for the acousto-optic cell are generated by four individual RF oscillators, designated f1, f2, f3, and f4 corresponding to the frequencies that are generated thereby. The RF voltage outputs of oscillators f1, f2, f3, and f4 are amplified by variable gain amplifiers A1, A2, A3, and A4, respectively. Driving AO-cell 22 with four frequencies provides four diffracted beams designated B1, B2, B3, and B4 corresponding to the driving frequencies. An undiffracted portion 16R of beam 16 is absorbed by a beam dump 26. The diffracted beams are directed by turning mirrors 27 and 28 into a folded optical path that is long enough to achieve a desired spatial separation of the beams. Once the beam separation is adequate, the beams can be focused by lenses 30 directly onto a workpiece. A beamsplitter 32 directs a sample of each beam to an individual detector to provide a measure of power in the beam. The samples are designated S1, S2, S3, and S4 corresponding to beams B1, B2, B3, and B4. Detectors are designated D1, D2, D3, and D4 corresponding to beams B1, B2, B3, and B4.
The detectors and associated circuitry 34 monitor power of each of the diffracted beams. The detector outputs are compared by a processor 36 against four input reference voltage signals provided by processor 36 in response to commands C1, C2, C3, and C4, corresponding to beams B1, B2, B3, and B4. The commands provided to the processor establish the desired amount of optical power in each of the beams. The reference voltage signals are representative of that desired power. Comparison of the reference voltages and the detector outputs provides gain commands G1, G2, G3, and G4 to amplifiers A1, A2, A3, and A4 respectively. The gain commands provide that the amplifiers increase or decrease the power of driving frequencies f1, f2, f3, and f4. There are, in effect, four control loops designated L1, L2, L3, and L4 corresponding to the four beams B1, B2, B3, and B4, respectively. The amplitude of each of the four beams B1, B2, B3, and B4 can be independently adjusted by varying the gain and accordingly the RF output voltage of amplifiers A1, A2, A3 and A4 respectively.
When the power of one of beams B1, B2, B3, and B4 is changed, absent any other action, power in the other beams will change because all of the beams share a common input (beam 16). This can be defined as a cross coupling between the beams. By way of example, if a voltage at one driving frequency is increased to diffract more light out laser beam 16 into a corresponding secondary beam, then power in the other three beams will be correspondingly reduced. An effect of this is that processor 36, particularly if control loops L1, L2, L3, and L4 all have about the same bandwidth, can attempt to restore power to the other beams, thereby causing power in one or more of the beams to oscillate. One method of avoiding this oscillation is to program controller 36 such that if a change in power in one the beams is requested, processor 36 suspends control of the other beams, thereby avoiding a competition between the beams for available power. This method, of course, restricts controlled operation of the four beams to applications in which the beams are not required to be simultaneously controlled.
Controlling beams B1, B2, B3, and B4 compensates for the above-described cross coupling, in a way that will allow the beams to be simultaneously controlled, can be accomplished by cooperatively controlling the power in laser output beam 16. The output power of an RF excited CO2 laser, as exemplified here, can be conveniently controlled by pulse width modulating (PWM) at a constant repetition rate or by pulse repetition frequency (PRF) modulating the input RF power into the discharge at a constant pulse width. Processor 36 can be programmed to keep track of the total power required by all four beams and to command controller 14 via another control loop L5 to raise or lower the power in output beam 16 in response to a requested change in power, in one or more of beams B1, B2, B3, and B4. This allows the beams to be controlled simultaneously.
A deficiency of the above described apparatus is that it must be located close to a workpiece to facilitate beam-delivery. FIG. 2 depicts an alternative apparatus 40 that overcomes this deficiency. Only differences between and additions to apparatus 10 are described to avoid repetition. Only sufficient detail of the beam-dividing portion of the inventive apparatus is depicted to highlight details of the modification.
In apparatus 40, beamsplitter 32 of apparatus 10 has been removed. Individual focusing lenses 32 of apparatus 10 are replaced in apparatus 40 by a single lens 42. The focal length of the lens is selected, cooperative with the optical path distance from the lens from the AO-cell 22, such that diverging beams B1, B2, B3, and B4 propagate parallel to each other. In addition, the lens focuses the beams to a beam waist position at which are arranged proximal ends 44A of optical fibers 44. The spacing of the proximal ends corresponds to the spacing of the beams such that each beam is focused into a corresponding one of the optical fibers.
It should be noted that the term “single lens” as used in this description and the appended claims means only that one lens focuses all secondary beams into the optical fibers. The lens itself, while depicted in FIG. 2 as only a single optical element, may include two or more elements for aberration correction, as is known in the art.
Hollow core optical fibers 44 are held in grooves 46 in a support platform 48. Platform 48 is preferably made from aluminum, but can be made from other metallic materials or from dielectric materials. Fibers 44 can be referred to functionally as beam-spreading or beam-separating fibers. Fibers 44 are curved such that distal ends 44B thereof are separated from each other by a distance greater than proximal ends thereof are separated from each other. The distal ends are preferably separated from each other sufficiently to allow a male fiber connector 50 to be attached to each one, leaving sufficient space that a female connector can be easily attached to each one of the male connectors, as discussed below. The hollow core is required because of the long wavelength, for example greater than about 9.0 micrometers (μm) of radiation in the beam.
Hollow-core optical fibers 52 are provided for transporting the beams from the spreading fibers to the remote location of the workpiece (not shown). Proximal end 52A of each fiber has a female connector 54 thereon for attaching the fiber to male connector 50 of the corresponding spreading fiber. Beams are delivered from distal ends 52B of fibers 52 to the workpiece. Beamsplitters 56 send samples S1, S2, S3, and S4, one for each beam, to detectors D1, D2, D3, and D4 respectively. The detectors are cooperative with monitoring circuitry 34 as described above with reference to apparatus 10 of FIG. 1.
It is particularly important in the arrangement of the present invention that beam-power be monitored after leaving the transport fibers as depicted in FIG. 2 and not at the output of AO-cell 22 as is the case in apparatus 10 of FIG. 1. A reason for this is as follows.
Unlike solid-core fibers used for transporting radiation at wavelengths from the ultraviolet (UV) through the near infrared (NIR), hollow-core fibers are particularly susceptible to losses at curves in the fibers and relatively small changes in the curvature, even changes occasioned by vibration, can cause a significant change in loss. While the spreading fibers 44 can be maintained in a fixed curvature by grooves 46 in block 48, it will usually not be practical to constrain transport fibers 52 to prevent small changes in curvature. Accordingly, as it is important that power of the beams on the workpiece is controlled, it is important that power monitoring for control purposes occurs after each beam leaves distal end 52B of the corresponding transport fiber 52.
A disadvantage of both of the above-described apparatuses is that simultaneous delivery of the beams and the use of analog circuitry sets a limit on the precision and flexibility with which beam parameters such as power and energy can be controlled. It may be useful in certain applications to provide more precise control of such parameters.